1. Field of the Invention
The invention relates generally to refractory burner nozzles used to fire high temperature furnaces such as those in glass melting furnaces. More specifically, the invention relates to stress-relieving mechanisms for a burner nozzle.
2. Background Art
Burner nozzles employed in high temperature furnaces, such as glass melting furnaces, are made of refractory materials that can withstand high operating temperatures, for example, of greater than 900xc2x0 C. without softening. In operations, combustible gases flowing through internal passages of the burner nozzle typically have a much lower temperature than a xe2x80x9chot facexe2x80x9d that is exposed to the combustion zone and operating temperature of the furnace. This situation results in relatively large temperature gradients across the burner nozzle. These large temperature gradients cause thermal stresses in the burner nozzle, which at high levels may be sufficient to fracture the burner nozzle. In general, compressive stress develops in the heated hot face portion and tensile stress develops in the cooler portion of the burner""s refractory body. The ultimate tensile strength of refractory materials is usually much lower in magnitude than their ultimate compressive strength. Thus, thermal stresses in refractory materials result in fracture cracks propagating from the cooler region toward the hot face.
FIG. 1 illustrates a burner nozzle design of the prior art, as described in detail in European Patent Application EP 0969249A2 (Snyder et al.) by Praxair Technology, Inc., filed Jun. 29, 1999. The burner is of a refractory construction with a substantially rectangular three-dimensional form, with three nozzle ports arranged in a fan-shape, terminating in the hot face of the burner, to produce a wide flame. Although this Patent Application shows slits on the side surfaces of a burner nozzle, the Patent Application does not disclose using slits in the hot face, nor does it teach the optimal placement or depth of side surface slits.
FIGS. 2A-2C show the types of fractures that are typically observed in burner nozzles. The fractures can be classified according to their relative orientation with respect to the longitudinal centerline of the burner nozzle. For example, the most common type of fracture, in burner nozzles of the kind described in the Praxair patent, is a so-called transverse fracture 1 as illustrated in FIG. 2A, since it transverses the longitudinal centerline of the burner. The fracture 3 shown in FIG. 2B is a longitudinal fracture. This type of fracture runs along the centerline of the burner, between from the colder region 5, the surface of the burner that is farthest from the furnace combustion zone (not shown), and the hot face 7. Fractures probably start in a high stress region (an area with a combined high temperature change over a small dimension and area change, such as the junction between a plenum and the discharge flow nozzles.) FIG. 2C shows a diagonal fracture 9, which is less common.
Although the scientific literature1 has touched upon the fact that thermal stresses in a refractory article can be reduced by decreasing the linear dimension of a section of the refractory article that is perpendicular to the thermal flux, the literature does not adequately discuss, not to mention effectively teach, how to optimize thermal stress reduction in the refractory article. Nor does the literature or relevant patents suggest where to locate stress relieving slits in the refractory article and how deep a slit should be. Therefore, we believe that we have discovered the optimal placement and depth for achieving the desired result of reducing or even eliminating thermal stresses and to prolong the useful lifetime of burner nozzles.
The invention relates in one aspect to the optimized placement and depth of stress relieving slits in a burner nozzle having a hot face, side surfaces, and a plurality of internal gas flow passages. The burner nozzle comprises a plurality of stress relieving slits oriented in at least two different directions, and a selected number of the slits formed in the hot face. In some embodiments, a selected number of the slits are formed in the side surfaces. In some embodiments, the burner nozzle further includes an internal plenum smoothly or fluidly connected to the internal flow passages. In some embodiments, the slits formed in the hot face have a depth of approximately 50% to 70% of the perpendicular distance from the hot face to a leading edge of the plenum. Stated in another fashion, in some embodiments, the slits formed in the hot face have a depth of approximately 10% to 75% of a length of a radius that bisects an angle formed by the longitudinal axes of two adjacent internal flow passages as they terminate in the hot face. In some embodiments, the slits formed in the side surfaces, relative to the hot face, are positioned approximately 30% to 50% of a length of the burner nozzle. The slits formed in the side surfaces have a depth of 20% to 50% of the thickness of the side surfaces.
Thermal stresses experienced by the burner nozzle are substantially reduced by at least 10%, relative to a burner that does not have a combination of: a plurality of stress-relieving slits, each having a predetermined depth, formed in the hot face, where the slits are positioned between adjacent internal flow passages, and at least one stress slit is formed in each side surface. In comparison to a burner having only stress slits formed in the side surfaces, the thermal stresses experienced by the burner nozzle are reduced by at least 15%, and to a burner having no stress slits, the thermal stresses experienced by the burner nozzle are reduced by at least 20%. In particular, the thermal stresses experienced by the burner in the roof and floor of a center internal flow passage, an outboard internal flow passage, or a plenum, and are all reduced by at least 10%, relative to a burner having only stress slits formed in the side surfaces. Moreover, by employing optimized placement of the stress-relieving slits, the useful lifetime of a burner nozzle is prolonged as a function of stress reduction by at least one order of magnitude.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.